Precise, reproducible quantitation of fibrinogen from plasma using thrombin to convert it to fibrin requires careful attention to the technics of clot formation and syneresis. The precision of the method outlined indicates a coefficient of variation of 1.2% with replicate samples. The coefficient of variation in sequential studies of a single purified bovine fibrinogen pool was 2.8% and for a lyophilized commercial plasma it was 4.7%.